An increasingly popular use of computing devices is to allow a user the ability to interact with their physical surroundings. For example, handheld computers can provide the user with information on topics of interest near the user, such as restaurants, copy centers, or automated teller machines. Similarly, laptop computers seek to allow a user to control devices located in close proximity to that laptop, such as a printer located in the same room. Additionally, specialized computing devices, such as navigational systems found in many automobiles seek to provide the user with directions based on the user's current location. A necessary element of all of these applications is the ability to determine the location of the user.
Current systems for locating a user rely on extensive, and expensive, infrastructure to support the system. One well known system for determining location is the Global Positioning System (GPS). GPS is a satellite based system in which the receiver, carried by the user, requires a signal from at least four satellites to determine the location of the user. A similar system, known as E-911, relies on signals from a user's cellular telephone to triangulate the user's position. Two methods currently relied on by the e-911 system use either the angle of the arrival of the user's signal, the time of arrival, or the time difference of arrival. Both the GPS system and the E-911 system require the installation and maintenance of expensive and dedicated infrastructure. GPS, for example, requires maintaining many satellites, and the E-911 system requires either the installation of directional antennas on the cell phone and base station, or specialized equipment at the base stations to provide time synchronization. In addition, the E-911 system may require the user to purchase a new cell phone. These systems also suffer from other drawbacks inherent in their design. For example, the GPS system does not work indoors or in locations, such as downtown areas, with significant obstructions to the signal paths between the user and the satellites and cellular telephone base stations. Similarly, the E-911 system requires that the cell phone be able to communicate with multiple base stations simultaneously.
Additional location determining systems can be used in confined areas, but they too suffer from the need to purchase and maintain dedicated infrastructure. One known system for determining the location of a user indoors is the Active Badge system, that uses infrared beacons and sensors located throughout an area to pinpoint the user's location. Additional systems rely on wireless communication between a transmitter and detectors placed throughout the indoor environment. Such systems suffer from the same complex infrastructure problem as GPS or E-911. In either case, dedicated detection equipment must be installed throughout the indoor area. Because of the amount of infrastructure needed for even small areas, such systems cannot be scaled well to larger areas, such as a whole town or metropolitan area. Additionally, some of these systems, such as infrared based systems, can be impacted by environmental concerns, such as direct sunlight.
In addition to the above problems, some of the known systems for determining a user's location suffer from privacy concerns. Specifically, the user's position is determined by infrastructure that is exclusively controlled by a central authority. Thus, the central authority could use the system to determine the location of a user even when that user did not wish to learn their location, or did not wish to be located. Furthermore, because all of these systems rely on a central architecture, they all leave the user vulnerable to system outages when the infrastructure required is not properly operated or maintained by the central authority. Often, the user is simply forced to rely on human interaction to determine their location.